Dark Skin Evolution in Early Humans: Revisiting the Skin Cancer Hypothesis Through Migration-Related Mismatch.

Cancer as a selective force in human evolution remains largely unexplored in the field of cancer evolution and ecology. In this review, we examine one such proposal about the evolution of skin pigmentation in ancestral humans. The skin cancer hypothesis posits that deaths from sun-induced skin cancers in part favored the evolution of darker skin in early hominins as they dispersed into savanna mosaics. Evolutionary mismatch, where migrants and albinos with fair or extremely pale skin settle in high-UVR environments, has been used to support the hypothesis, but only the case involving albinos has been subject to examination and discussion. Using current data on skin cancer rates in Australians, we test the case of migration-related mismatch and find that, although skin cancers with metastatic potential are common, their typical onset occurs after reproductive age. This suggests that the protective effects of dark skin in ancestral humans were likely selected not for protection against skin cancers, but for mitigating other UV-related risks, e.g. maintaining thermoregulation and water barrier integrity. We then discuss several ad hoc explanations, and their criticisms, to preserve the skin cancer hypothesis, concerning melanoma as a selection pressure, life history traits of ancestral humans, and the grandmother effect. We conclude that lethal skin cancers may have shaped human evolution indirectly, insofar as post-reproductive elderly contributed to ancestral social structures by providing alloparental care to their kin.